Scouting time
Occasionally rootworms can cause problems when the corn follows very weedy soybeans or soybeans infested with volunteer corn that persisted through July. In some upper Midwestern states, northern corn rootworms can survive a non-corn crop by remaining in the egg stage for two winters before hatching, but this phenomenon has not been confirmed within Kansas. In another form of 'rotation resistance', eggs may be laid in soybean fields in late summer where this crop is usually followed by corn. Western corn rootworms, by far the state’s most significant species, are generally not known to lay many eggs in non-corn fields in Kansas, so crop rotation still works well here.
Thresholds
Rootworm insecticides are rarely needed in first year corn fields. Rootworms are less of a problem in sandy soils and in southeast Kansas, south of U.S. Highway 54. Previous year counts of rootworm beetles can be used to establish the potential for economic damage and the need for insecticide protection the next growing season. Insecticides are best applied before rootworm damage has become severe (mid-May through mid-June), sometimes before or close to egg hatch. Adequate precipitation or soil moisture must be available to move the products into the rootworm feeding zone. Control may be improved if the insecticide is cultivated into the soil. Some materials are registered for chemigation.
Adult beetles may prevent pollination by early silk clipping, but clipping after pollination does not affect yield. Foliar spray treatments are probably justified if there are eight to 10 beetles per plant and 10 percent of the silks are beginning to show.
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